1. Field of Invention
The invention relates to a method for preparing an alignment mark on a substrate and, in particular, a method for distinguishing one feature pattern from another feature pattern in a multi-pattern structure.
2. Description of Related Art
In material processing methodologies, such as those used in the fabrication of micro-electronic devices, pattern etching is often utilized to define the intricate patterns associated with various integrated circuit elements. Pattern etching comprises applying a patterned layer of photo-sensitive material, such as photo-resist, to a thin film on an upper surface of a substrate, and transferring the pattern formed in the layer of photo-sensitive material to the underlying thin film by etching.
The patterning of the photo-sensitive material generally involves coating an upper surface of the substrate with a thin film of photo-sensitive material and then exposing the thin film of photo-sensitive material to a pattern of radiation by projecting radiation from a radiation source through a mask using, for example, a photolithography system. Thereafter, a developing process is performed, during which the removal of the irradiated regions of the photo-sensitive material occurs (as in the case of positive-tone photo-resist), or the removal of non-irradiated regions occurs (as in the case of negative-tone photo-resist). The remaining photo-sensitive material exposes the underlying substrate surface in a pattern that is ready to be etched into the surface.
Photolithography systems for performing the above-described material processing methodologies have become a mainstay of semiconductor device patterning for the last three decades, and are expected to continue in that role down to 32 nm resolution, and less. Typically, in both positive-tone and negative-tone pattern development, the minimum distance (i.e., pitch) between the center of features of patterns transferred from the mask to the substrate by a photolithography system defines the patterning resolution.
As indicated above, the patterning resolution (ro) of a photolithography system determines the minimum size of devices that can be made using the system. Having a given lithographic constant k1, the resolution is given by the equationr0=k1λ/NA,  (1)
where λ is the operational wavelength of the EM radiation, and NA is the numerical aperture given by the equationNA=n·sin θ0.  (2)
Angle θo is the angular semi-aperture of the photo-lithography system, and n is the index of refraction of the material filling the space between the system and the substrate to be patterned.
Following equation (1), conventional methods of resolution improvement have lead to three trends in photolithography technology: (1) reduction in wavelength λ from mercury g-line (436 nm) to the 193 nm excimer laser, and further to 157 nm and the still developing extreme-ultraviolet (EUV) wavelengths; (2) implementation of resolution enhancement techniques (RETs) such as phase-shifting masks, and off-axis illumination that have lead to a reduction in the lithographic constant k1 from approximately a value of 0.6 to values approaching 0.25; and (3) increases in the numerical aperture (NA) via improvements in optical designs, manufacturing techniques, and metrology. These latter improvements have created increases in NA from approximately 0.35 to values greater than 1.35.
Immersion lithography provides another possibility for increasing the NA of an optical system, such as a photolithography system. In immersion lithography, a substrate is immersed in a high-index of refraction fluid (also referred to as an immersion medium), such that the space between a final optical element and the substrate is filled with a high-index fluid (i.e., n>1). Accordingly, immersion provides the possibility of increasing resolution by increasing the NA (see equations (1) and (2)).
However, many of these approaches, including EUV lithography, RET lithography, and immersion lithography, have added considerable cost and complexity to photolithography equipment. Furthermore, many of these approaches continue to face challenges in integration and challenges in extending their resolution limits to finer design nodes.
Therefore, another trend in photolithography technology is to utilize a double patterning approach, which has been introduced to allow the patterning of smaller features at a smaller pitch than what is currently possible with standard lithographic techniques. One approach to reduce the feature size is to use standard lithographic pattern and etch techniques on the same substrate twice, thereby forming larger patterns spaced closely together to achieve a smaller feature size than would be possible by single exposure. During double patterning, a layer of photo-sensitive material on the substrate is exposed to a first pattern, the first pattern is developed in the photo-sensitive material, the first pattern formed in the photo-sensitive material is transferred to an underlying layer using an etching process, and then this series of steps is repeated for a second pattern, while shifting the second pattern relative to the first pattern. Herein, the double patterning approach may require an excessive number of steps, including exiting the coating/developing tool and re-application of a second layer of radiation-sensitive material.
The aforementioned double patterning technique may be referred to as a Litho-Etch-Litho-Etch (LELE) technique. Other double patterning techniques, such as Litho-Litho-Etch (LLE) or Litho-Freeze-Litho-Etch (LFLE) have been developed to improve throughput by reducing the number of back-and-forth operations between photolithography and etch equipment. In the former, a single layer of photo-sensitive material is imaged twice with the first pattern and the second pattern in the photolithography system, the patterns are developed, and then the patterns are transferred into an underlying layer using etching techniques. In the latter, a first layer of photo-sensitive material is imaged and developed with a first pattern, the first pattern is chemically frozen, a second layer of photo-sensitive material is applied over the chemically frozen, patterned first layer of photo-sensitive material, the second layer of photo-sensitive material is imaged and developed with a second pattern, and then the patterns are transferred into an underlying layer using etching techniques.
Another approach to double the resolution of a lithographic pattern is to utilize a dual tone development approach, wherein a layer of photo-sensitive material on the substrate is exposed to a pattern of radiation, and then a double pattern is developed into the layer of photo-sensitive material by performing a positive-tone development and a negative-tone development. However, current dual tone development approaches lack the ability to adjust, control, and/or optimize the double pattern formed on the substrate.
As evidenced above, multi-patterning techniques, such as double patterning, have become a common technique used to push the limits of optical lithography. Although many approaches exist, the most cost effective technique is litho-litho-etch (LLE) or Litho-Freeze-Litho-Etch (LFLE). Both techniques allow formation of the double pattern on the substrate in the photolithography system prior to use of any etch equipment.
However, one issue that arises when performing double patterning of a substrate includes discernment between the first pattern (L1) and the second (L2) after full double patterning. This discrimination of patterns becomes particularly challenging within a large exposure field or array of lines/spaces (L/S). Typically, an alignment mark is placed within the field to discern between the first and second patterns, L1 and L2. Large openings within an array have been used. However, these conventional alignment marks have suffered from problems with patterning and defectivity. Therefore, an alignment mark is needed which is robust in the patterning process and is accurate in discerning the first pattern from the second pattern.